Modern automatic washing appliances such as dishwashers and clothes washing machines commonly employ electromechanical timers for controlling various cyclical appliance operations. Typically, timing accuracy is subject to relatively high tolerance variations. For appliances employing time-controlled fill cycles, high tolerance variations can result in excess water usage beyond that required for satisfactory washing performance. For example, on a 78 minutes per revolution timer cam such as typically employed in a dishwasher, the fill time interval may have up to a plus or minus 8 seconds fill time variation because of the many interacting tolerances in the timer. Nominal dishwasher appliance fill flow rates are on the order of 1.75 gallons/minute. At such rates, a time variation of plus or minus 8 seconds results in a variation of plus or minus roughly 2 pints of water during each fill cycle. For a dishwasher having 6 fill cycles for each load of dishes, the variation in total water usage per load is on the order of plus or minus roughly 1.5 gallons.
One technique known in the art for improving the accuracy of such timers is to employ a drop-to-start, drop-to-stop arrangement. In such an arrangement each resilient contact arm carries a cam follower biased against the control cam track. The control switch is made when the first cam follower drops and opens when the second cam follower drops. Such an approach eliminates the rise portion of the track profile and the gap between contact arms and the relative positioning of cam and switch block as sources of tolerance variations. However, even with this double drop approach, positioning of each cam follower on its corresponding contact arm and the relative positioning of the contact arms themselves remain as significant sources of tolerance variations.
In order to reduce water usage in washing appliances such as dishwashers, an electrical mechanical timer of relatively simple construction which provides improved accuracy capability, particularly for short intervals, is needed.
By this invention there is provided an improved electromechanical timer for washing appliances which improves the timing accuracy for short intervals.